Eat more meat. The protein will help build up your muscles to support you. Porkchops, steak, bacon, just eat it as much as possible.
lol
Don't give health advice, ever.
Increasing protein and continuing to live a sedentary lifestyle will only create more problems, unless, for some exceptionally rare reason, you have a protein deficiency of some style.
Protein doesn't build muscle. Protein is used by the body to repair damaged muscle tissue after you have torn said muscle tissue. Muscle tissue doesn't tear unless it is strenuously abused, which requires a decent amount of effort.
Muscle mass increases only during/after the use/tear/repair/build phase. If the muscle is not actively getting repaired, there is no chance of muscle mass gain.
If you are gaining weight by doing nothing and increasing protein count, you are gaining fat weight (protein can be stored as fat), and if consuming excess protein, there exists a chance you may harm yourself in other ways (long-term).
Anyhow, what others have stated is pretty much the better advice: get up and move around.
Also, try to adjust your sitting position from time to time. If you change it from time to time, you shift the weight onto different parts of your body and, throughout the course of the day, average it out across a larger zone (with less time focused on a single area).
If you can change the angle of the seat back (relative to seat bottom), adjust height, and adjust tilt... do so once every hour or so.
Also, use one of those breaks (5-10 minutes every hour or two) as a moment to get a little core stability training into your day.
Core Workouts
If they are very difficult, build up toward a minute over time. If you can do a minute, great.
That article gives great tips, and perfect examples.
I haven't read it all, so some might not be well-suited. Generally, their fitness info is spot-on for physiological fitness and nutritional purposes; there is a ton of misinformation out there on these subjects, and some major players have terrible advice (for some it's rare, other places being right is rare).
I can't say whether that article speaks of simple body aches due to posture, but even without building visible muscle mass in the core, you definitely can improve the specific areas of discomfort you would get from sitting (good or bad posture) most of the day.
edit:
The human body isn't exactly best suited for sitting the way we do, period. We may be upright, but sitting isn't the body's best friend. We have tricks to help keep the spine, hips, and legs in the most neutral positions possible, but there are many factors at the individual level that sometimes means it doesn't work out so well. Some people also just have an anatomy that, regardless of proper support, will be destined for discomfort/pain in the future.